The political battle over a controversial bill to scrap by-elections intensified last night as the government's big guns were rolled out to counter growing calls for the withdrawal of the proposed amendment.

As pan-democrats stepped up their calls for people to take to the streets in tomorrow's July 1 march in protest over the bill, Secretary for Justice Wong Yan-lung and Secretary for Constitutional Affairs Stephen Lam Sui-lung issued a staunch legal defence of a revised proposal which would see Legislative Council seats vacated midterm filled - in most cases - by people on the same party list as the ex-incumbent.

The concession came after Beijing's liaison office earlier this week suggested the Hong Kong administration's original plan should undergo big changes.

Wong cited a number of legal precedents to bolster the government's position that the bill in its revised form is in line with the Basic Law. He also said that the government had the discretion to decide the method for filling midterm vacancies in Legco, citing a Court of Final Appeal judgment.

The pair's public appearance was a response to the Bar Association's fourth warning that the proposed amendment was unconstitutional.

Their move came after Chief Executive Donald Tsang Yam-kuen spoke about the plan, defending it in public for the first time. 'I hope citizens will understand and support the proposal,' he said.

The original plan to award the seat to the next-placed candidate overall was introduced to stop stage-managed or politically inspired resignations, such as those last year when five lawmakers quit their seats to trigger by-elections they hoped would be a referendum on democracy.

But the administration is facing challenges even from its own allies on the revised bill. A day after it announced the U-turn to allow the next-placed candidate on the same ticket as the departing member to fill a midterm vacancy, instead of the next overall, 13 pro-government and independent lawmakers said yesterday they were still undecided on which way to vote.

Former executive councillor David Li Kwok-po, normally a staunch government supporter, was among those who expressed reservations. Asked whether he accepted the new amendments, the financial sector lawmaker said: 'It is quite difficult at the moment. 'Another Beijing-loyalist lawmaker Lam Tai-fai, of the industrial sector, said: 'What troubles me most is the legality of the proposal - the Bar Association says it is unconstitutional while the government says it is lawful.'

Bar Association chairman Kumar Ramanathan said: 'We have to consider the proposal as a whole ... the elements of the old proposal were retained in the revised mechanism, the constitutional problem remains. We regret if the government continues to press the proposal.'

Pan-democrats are mobilising people - especially voters in Legco's seats for trades and professions - to put pressure on their representatives not to back the bill, which they called 'malicious legislation'.

'Citizens should take to the street and force the government to withdraw this rude law,' said Democratic Party lawmaker Cheung Man-kwong. 'All voters, including those of functional constituencies, should pressurise their legislators for opposition - in a domino-effect style.'

Civic Party legislator Audrey Eu Yuet-mee urged voters to appeal to their Legco representatives not to support bill.

'Voters should write or e-mail or see their legislators and tell them they have no right to deprive citizens of the right to vote,' she said.

Professional organisations and pressure groups also urged people to join the July 1 march. One of them, the Hong Kong Journalists Association, said: 'The replacement mechanism proposed by the government will deprive citizens' rights to vote in by-elections, thus harming freedom of expression.'

By last night 700 people had signed an online petition initiated by academics, urging the bill's withdrawal.